The invention relates to compressable fluid type spring devices for vehicles, but more particularly, the invention relates to airsprings of the type with a flexible type rubber sleeve with a rolling lobe portion.
Rolling lobe type airsprings are well known in the art and are made with a sleeve having a chamber portion connected to a closure member and an inverted rolling lobe portion connected to a piston that partially reciprocates in the chamber portion of the sleeve. The general formula for calculating spring rate of such an airspring is well known and documented such as in U.S. Pat. No. 4,629,170. In general, the spring rate of a conventional airspring can be represented by the following equation: ##EQU1## where: K=spring stiffness
P=absolute internal pressure PA1 Pg=gauge pressure PA1 A.sub.e =effective area acted on by air pressure PA1 V=air volume PA1 n=ratio specific heats for air PA1 dA.sub.e /dH=change in effective area with spring height
An airspring is a load support member that utilizes the compressable characteristics of air for a springing effect. From the foregoing equation it is seen that spring rate may be changed by altering the pressure in the airspring, but the corresponding change in spring rate also changes the load carrying capability of the airspring. Another conventional manner for changing the spring rate of an airspring is to change the effective area that is acted on by the internal pressure of the spring. This is done by altering the external shape of the piston which laterally supports part of the rolling lobe portion of the sleeve. There is no change in effective area or spring rate if the piston is straight sided or cylindrical. However, a reduced effective area is achieved by a frustoconically shaped piston that reduces in size as it enters the chamber portion; and an increase in spring rate is achieved by a frustoconically shaped piston whose effective area increases as it enters the chamber portion.
Another method for changing the effective spring rate of an airspring is to increase its volume by increasing the inflated cylindrical diameter of the sleeve portion or to increase the inflated cylindrical length of the sleeve; both methods substantially affect the space envelope of the airspring. An increase in the inflated diameter also has the effect of increasing spring rate by an attendant increase in effective area. Of course, spring rate can also be influenced with the use of external volume reservoirs to increase volume and thereby reduce spring rate.
As a practical solution, spring rates for any particular automotive application are more easily and economically adjusted by varying the contour of the piston because of its direct effect on effective area. Increasing the cylindrical diameter of the sleeve per se to change volume and reduce spring rate becomes impractical because an increase in diameter increases the effective area which operates to further increase spring rate. Increasing the length of a sleeve to increase volume while maintaining effective area becomes impractical because it substantially affects the height of the airspring while also increasing its tendency to buckle. An example of a sleeve with a contoured piston that affects spring rate is shown in Canadian Pat. No. 1,125,319. An example of an airspring having a sleeve with increased length and a contoured piston is shown in U.S. Pat. No. 4,174,827.
In practice, the effects on spring rate caused by a piston entering and exiting an air chamber of a rolling lobe pneumatic spring, are routinely compromised by external means such as external volume reservoirs, pressure relief mechanisms, and pumps that affect pressure and volume; piston effects on spring rate are routinely compromised by internal means such as piston contour and secondairily, by sleeve length and sleeve diameter. An increase in length to increase volume has the disadvantage of increasing height and thereby decreasing buckling resistance, while an increase in diameter to increase volume has the disadvantage of increasing spring rate by increasing the effective area. The combined effects of length and diameter of an inflated cylindrical sleeve establish an optimum spring rate for a given space envelope and load. Consequently, contouring a piston has become substantially the only internal means for compromising piston effects on spring rate.
Sleeves for airsprings are usually of the elastomeric type that are reinforced with successive layers of fibrous members oriented at opposite helical angles with respect to the sleeve; the fibrous members may be in the form of helically disposed cords. An example of making a rolling lobe type sleeve is disclosed in U.S. Pat. No. 3,666,598 where uncured extruded elastomeric tubing is slipped over a thin walled mandrel forming a liner. Rubberized cord fabric, cut in the form of a parallelogram, is helically wound over the liner in one direction. A second layer of rubberized cord fabric is wound in the opposite direction over the first layerd cord fabric at substantially the same, but opposite, helical angle. A cover is applied over the cord and the assembly is cured.
The sleeve is reinforced with successive layers of embedded cords that are disposed at opposite helical angles of about 36 degrees. After the sleeve is attached to a piston and an end enclosure, it is pressurized which "pantographs" the successive layers of cord to usually a larger helical angle that approaches a locking helical angle of about 66 degrees; the increase in angle causes the sleeve to increase in diameter at the chamber portion of the sleeve.
In some methods of making a sleeve, a frustoconical mandrel is inserted into a portion of the sleeve and cured thereon so that the chamber portion of the sleeve has a larger diameter to facilitate attachment to an end closure member. The frustoconical mandrel has the effect of altering the helical angle of the cord while also changing the cord spacing; when inflated, such sleeves inflate to form substantially a cylindrical member.